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PROTON REACTION ANALYSIS FOR LITHIUM AND FLUORINE I N GRAPHITE, USING A SLIT SCANNING TECHNIQUE R . L. Macklin, J. H. G i b b o n s , and T. H. Handley

NOTICE This document contains information of a preliminary nature and was prepared primarily for internal use a t the Oak Ridge National Laboratory. It i s subject to revision or correction and therefore does not represent a final report.

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ORNL-'IN-2238

C o n t r a c t N o . W-7405-eng-26

Physics D i v i s i o n

PROTON REACTION ANALYSIS FOR LITHIUM APJD FLOURINE I N GRAPHITE, U S I N G A SLIT SCANNING TECHNIQUE R . L. M a c k l i n , J. H. G i b b o n s , and T. N. H a n d l e y

JULY 1968

OAK R I D G E NATIONAL M O R A T O R Y

Oak R i d g e , T e n n e s s e e O p e r a t e d by UNION CARBIDE CORPORATION for the U . S . ATOMIC ENERGY COMMISSION

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iii

CONTENTS Page Abstract

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... ... ... . .. Results and Discussion ... Microscopic Examination of Sample . . References . . . . . . . . . . . . . . ..... Method and Apparatus . Standards . . . . . . Sample . . . . . . . . Introduction

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PROTON REACTION ANALYSIS FOR LITHIUM AND FLUORINE IN GRAPHITE,

USING A SLIT SCANNING TECHNIQUE R. L. Macklin, J. H. Gibbons, and T. H. Handley ABSTRACT Protons from the ORNL 3 MV Van de Graaff accelerator were brought to a line focus and collimated thru slits of either 0.025 or 0.0075 cm. Cross sectional cuts of graphite samples were moved across the beam to study the distribution of Li and F with depth by measuring the yields of neutrons and of g a m a rays from the reactions TLi(p,n) and lgF(p,q). A graphite sample exposed to molten fluorides in a loop experiment at the Oak Ridge Research Reactor showed about 20 and 100 ppm (by weight) Li and F respectively, 2 m below the exposed surface. The observed ratio of Li to F was close to that characteristic of the molten salt down to 3 mm below the exposed surface.

INTRODUCTION The proton induced reactions 7Li(p,n) and ''F(p,q)

have been

1 used to measure the concentration of the target nuclides in graphite. This can be done in the presence of considerable radioactivity from fission products, making the method attractive for studies of penetration in graphite moderated molten salt reactors.2

In the previous work

the exposed surface of a sample was ground away a layer at a time and the concentrations near each freshly exposed surface measured with a 0.32 cm diameter, 0.1-1microampere proton beam.

This process required

special handling for the radioactive material ground off and introduced considerable delay in the measurements.

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METHOD AND APPARATUS

The ORNL focussed to a

3 MV Van de Graaff accelerator proton beam is normally 1 mm diameter spot at the target with a magnetic

quadrupole strong focus lens. By deliberately detuning this lens the beam can be brought to a horizontal (or vertical) line focus about 1.0 x 0.08 cm. By using several microamperes of current and a metal slit, sufficient beam could be passed through to a sample to perform the analysis over an area 0.0075 cm x 0.60 cm, or with better sensitivity (compared with background from F impurities in the slit collimator material) 0.025 cm x 0.60 em.

With this arrangement, a sample exposed

in a reactor can be sectioned, cutting perpendicular to an exposed face and the distribution of 7Li and

with depth studied by simply moving

the sample across the proton beam.

The apparatus we used for this is

indicated schematically in Fig. 1. The graphite sample was mounted on a micrometer head and moved in to intercept the beam.

The proton beam

passing the sample induced a blue fluorescence in the quartz viewing plate at the left so the point at which the edge of the sample just intercepted all of the protons could be easily noted.

The profile of

the beam passing through the slit was measured with a 0.005 cm A1 foil glued to the end of the graphite blank. 27Al(p,y)

Figures 2 and 3 show the

yield from this foil using 0.025 ern and 0.0075 cm gold slits

respectively. Slits of tantalum showed too much contamination with fluorine. The slight asymmetry of the scans is probably due to a few protons striking the exposed face of the A1 foil rather than the edge as the blank sample was pulled up out of the beam.

2. I

- 3 Gamma r a y s were d e t e c t e d by a 12.5 cm d i a . x 17.5 cm long N a I ( T 1 ) c r y s t a l through a 1.25 ern lead f i l t e r a t t h e s i d e of t h e sample housing. Neutrons were d e t e c t e d i n t h e s t r a i g h t ahead p o s i t i o n as i n r e f e r e n c e (1). A s t h e g r a p h i t e b l a n k sample could r e a d i l y be brought i n t o t h e beam t o

check t h e background without dismounting t h e sample, we used 2.06 MeV p r o t o n s ( j u s t below t h e ’Be(p,n)

t h r e s h o l d ) throughout, r a t h e r than

dropping t h e energy below t h e “ L i ( p , n ) t h r e s h o l d (1.88 MeV) t o check background.

STANDARDS P r e s s e d samples of g r a p h i t e powder c o n t a i n i n g weighed q u a n t i t i e s

of 7LiF were used as s t a n d a r d s .

SAMPLE The sample s t u d i e d w a s taken from a molten s a l t convection l o o p exposed i n t h e Oak Ridge Research Reactor ( O m ) l a s t year.’ a v a i l a b l e for s t u d y i s shown i n F i g .

The p i e c e

4 (ORNL S l i d e No. 74245).

The

s t r a i g h t e s t s i d e shown i s t h e w a l l of a molten s a l t e x i t channel s e c t i o n e d a x i a l l y a f t e r exposure.

The proton b e a m w a s c e n t e r e d a l o n g

t h i s s u r f a c e , as n e a r l y p a r a l l e l t o it as i t s i r r e g u l a r i t i e s would permit.

The r e a c t o r exposure h i s t o r y of t h e sample included over 1019

f a s t neutrons p e r square c e n t i m e t e r

(79% of it

w i t h 235U b e a r i n g molten

s a l t ) and a cracked o u t l e t p i p e w i t h consequent a i r contamination. RESULTS AND DISCUSSION The homogeneity o f t h e s t a n d a r d s w a s d i s a p p o i n t i n g .

Scans of

o r i g i n a l and f r e s h l y c u t s u r f a c e s showed c o n s i d e r a b l e v a r i a b i l i t y .

The

s t a n d a r d d e v i a t i o n a t a s i n g l e proton beam p o s i t i o n appeared t o b e about

-4W

3546.

An uncertainty of that size is indicated for the absolute scales

of concentration. The standardization was, however, based on the grand average of all our measurements on the 200 ppm ( 7LiF by weight) standard. Until more uniform standards are available or the cause of the observed nonuniformity is better understood, we hesitate to claim greater accuracy for the concentration scale. The fluorine and lithium concentrations in the sample as a function of distance from the flow channel surface are shown in Figs.

5 and 6. The rapid decrease in concentration in the first ten mils or so (0.025 cm) is expected. The persistence of moderate concentrations

to much greater depths is puzzling. Li concentration.

Figure

7 shows the ratio of F to

The ratio is persistently near the ratio characteristic

of the molten salt mixture rather than that for the LiF molecule or the progressively lower values expected for free ionic diffusion.

It has

4 that the bulk of the material seen at depth may repre-

been suggested

sent liquid phase intrusion via a slant crack.

It should be noted that

the promotion of graphite wetting by the air contamination experienced in the ORR Loop should make smaller cracks than usual effective in this regard. MICROSCOPIC MAMINATION OF SAMPLE

The graphite sample was viewed (x 10 and x 20 magnification) with a binocular microscope.

The surface is relatively quite rough,

with saw markings a few thousandths of an inch deep and pock-marked with many voids several thousandths of an inch in diameter.

There is evidence

of at least one long groove or crack in the sample at about 20' inclination to the exposed surface. The surface is also irregularly

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discolored, reminiscent of differential heating effects. When wetted with acetone numerous bubbles rose to the surface, clearly implying penetration of the liquid into sub-surface voids.

In short, this sample

shows gross irregularities and imperfections compared to the sample 1

taken from the MSRE that we studied earlier.

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REFERENCES 1. R. L. Macklin, J . H. Gibbons, E. R i c c i , T. Handley, and D. Cuneo, t o be published i n Nuclear A p p l i c a t i o n s . 2.

H. G. MacPherson, Power Englneering, January, 1967, p. 2-8.

Molten-Salt Reactor Program Semiannual Progress Report f o r P e r i o d Ending August 31, 1967. ORNL-4191, P a r t 1 5 .

E. G. Bohlmann, p r i v a t e communication.

- 7 FIGURE CAPTIONS Fig. 1

Schematic side view of the scanning system.

The graphite sample

(from a specimen exposed to molten salts containing 7Li and

19F) is cut so that the exposed surface is uppermost in the figure. The 7Li(p,n) and 19F(p,q) yields indicate concentration as a function of depth as the sample is moved up across the collimated proton beam. Fig. 2

Proton beam profile using the .O25 ern slit-collimator as measured by the 27Al(p,y)

reaction yield from a .005 em A1 foil

seen edge-on. The small random deviation of the points from the curve is largely an indication of mechanical precision and reproducibility in the experiment (about 0.0003 em). Fig. 3

Proton beam profile for the .0075 em slit-collimator measured using the .OO5 em A l foil. At the left the beam is hitting clean graphite, whereas at the right it can graze tangentially the flat face of the A1 foil (see Fig. 1) producing a slight tail in the composite resolution function shown.

Fig.

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L o c a t i o n and o r i e n t a t i o n of t h e sample c u t from t h e

1967 ORR

Loop Specimen. The molten salt flow was largely upwards through the drilled holes and channels shown in the photograph. Fig.

5 - Fluorine concentration as a function of depth in the

ORR Loop

graphite sample. The data taken at two slit widths are selfconsistent.

The results cannot be accounted for by simple

ionic diffusion and may reflect non-homogeneous structure of the graphite. Fig.

6 - Lithium concentration as a function of depth in the ORR Loop graphite sample. The data obtained with the two slit-collimators

-aare self-consistent. The lithium concentration beyond 100 mils depth decreases more rapidly than expected. Fig.

7 - Mass concentration ratio, F/Li, versus depth. The closeness of the ratio observed to that typical of the bulk salt suggests that most of the material found (at depths down to 30 mils) may represent a liquid intrusion.

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ORNL-DWG 68-655f

MICROMETER DRIVE MOUNT CLEAN GRAPHITE FOR BACKGROUND CHECK

0 . 0 0 5 c m AI FOIL FOR SCANNING BEAM PROFILE

GAMMA RAYS

CONVERGING PROTON BEAM (2.06 MeV) ANEUTRONS

0.0075 x 0 . 6 0 c m COLLIMATING SLIT

SAMPLE TO BE SCANNED MICROMETER DRIVE MOUNT

Fig. 1

ORNL- DWG 68-6552

6000

5000 W

2 4000 k

8 3000 W

$ -J

2000

--I----

io00

DISTANCE (mils)

Fig. 2

ORNL- DWG 68- 6553

4 0,000

8000

6000

4000

2000

20 22 DISTANCE (mils) Fig. 3

ORNL- DWG 68-6736

SURFACES OF MOLTEN SALT FLOW CHANNELS CROSS SECTIONAL SURFACE CUT AFTER EXPOSURE

-------_-_- -_ - -- -- ---------a-- - - -

PROTON B E A M POSIT10 AT VARIOUS DISTANCE FROM EXPOSED SURFACE /

COLLIMATED PROTON B E A M

Fig. L

ORNL-DWG 68-6556

40,000 5000

2000 1000

500

sp In m +I LL

200

100

5 10 20 50 100 DEPTH BELOW SURFACE (mils) Fig. 5

200

500

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ORNL-DWG 68-6554

io00

500

200

-400 E

3 50 6 In rr)

.- 20 -I

(0 5

' 4

! ,

I I

1 , l . l

1 1 1 1 'I

50 400 20 DEPTH BELOW SURFACE (mils)

Fig. 6

200

500

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ORNL-DWG

68-6555

I000 500

200

F a

-I

\ 2 0

io 20 50 100 200 DEPTH BELOW SURFACE (mils) Fig. 7

500 1000

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